This invention relates to ringing signal detection circuits for subscriber's apparatus each being used together with a telephone set for an office line.
In the present specification, the terms of the subscriber's apparatus includes a facsimile device, a telephone automatic answering device, a key-telephone-system control unit, a direct connection telephone set and all other apparatus or devices which has ringing signal detection circuits and which are used together with telephone sets to common office lines.
In those apparatus, when a dialling device is operated at the attached telephone set for calling another party, the ringing signal detection circuit of the subscriber's apparatus and the attached telephone set are connected to a common office line. Therefore, the dial pulse signal sent out from the attached telephone set is also applied to the ringing signal detection circuit of the apparatus.
For example, the key-telephone-system control unit has a ringing signal detection circuit which is connected to an office line. When a ringing signal is detected on the office line, the control unit operates to drive a bell, indicator lamps and other devices for noticing the call. When one of key telephone sets in the system is connected to the office line for calling another subscriber, the telephone set is also connected to the ringing signal detection circuit. Therefore, a dial pulse signal sent out from the telephone set to the office line is also applied to the ringing signal detection circuit.
A facsimile device is also connected to a telephone office line together with a telephone set. In reception of a call by another party, the apparatus detects the ringing signal incoming on the telephone line and automatically responds to the call. When calling another subscriber's apparatus, the subscriber operates a dialling device of an attached telephone set to send out the dial pulse signal to the telephone office line. Then, the dial pulse signal is also applied to the apparatus connected to the telephone office line together with the telephone.
In order to prevent such a subscriber's apparatus to erroneously operate in response to the dial pulse signal sent out from the attached telephone set connected to the office line together with the subscriber's apparatus, the ringing signal detection circuit of the subscriber's apparatus must be provided with a function for distinctly distinguishing the ringing signal from the dial pulse signal.
In many public telephone systems, a sinusoidal AC signal is used for the ringing signal. The sinusoidal ringing signal has usually a frequency of 16 Hz and a voltage of, typically, 65 volts (V). The ringing signal is sent out for a time period of one second and thereafter pauses for a time period of two or three seconds, and the sending for one second and the pause for two seconds are repeated.
While, the dial pulse signal is generated by switching on and off the office telephone line, to which a DC voltage of 48 volts (V) is usually applied, by operation of the dialling device of the telephone set. The repetition frequency of the dial pulse signal is usually 10 Hz.
As described above, since the ringing signal and the dial pulse signal are different in voltage level, a known ringing signal detection circuit is made to detect the voltage of the ringing signal.
However, when the dial pulse signal arrives at a telephone office through the telephone office line, an electromotive force is induced due to an inductance of the exchanging unit in the telephone office. The induced voltage signal is superposed on the dial pulse signal and is applied to the calling party's subscriber's apparatus. Since the voltage of the dial pulse signal on which the induced voltage signal is superposed is nearly equal to the voltage of the ringing signal, the detection circuit of the calling party's subscriber's apparatus may erroneously detect a ringing signal.
In another known method, a capacitor is used in the detection circuit so as to be charged by the incoming signal. Since the ringing signal and the dial pulse signal are different from one another in the voltage and the frequency, capacitor voltages charged by respective signals. Therefore, the ringing signal is detected and distinguished from the dial pulse signal by the capacitor voltage as charged. However, since the capacitor is not always charged to a constant level by ringing signals and since the capacitor voltage charged by the ringing signal is often nearly equal to the capacitor voltage charged by the dial pulse signal due to the above-described induced voltage signal, it is quite difficult to predetermine a capacitor voltage level for identifying the ringing signal.